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Segments Disclosures - Reported Segment Earnings (Loss) and Segment Assets (Details) - CAD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Disclosure of operating segments [line items] |  |  |  |
Revenues |  | $ 2,101 | $ 2,347 | $ 2,249
Fuel, carbon compliance and purchased power |  | 968 | 1,086 | 1,100
Gross margin |  | 1,133 | 1,261 | 1,149
Operations, maintenance and administration |  | 472 | 475 | 515
Depreciation and amortization |  | 654 | 590 | 574
Asset impairment |  | 84 | 25 | 73
Gain on termination of Keephills 3 coal rights contract | $ 88 | 0 | 88 | 0
Taxes, other than income taxes |  | 33 | 29 | 31
Termination of Sundance B and CPPAs |  | 0 | 56 | 157
Net other operating expense (income) |  | (11) | (49) | (47)
Operating income |  | (99) | 335 | 160
Equity income from associate |  | 1 | 0 | 0
Finance lease income |  | 7 | 6 | 8
Net interest expense |  | (238) | (179) | (250)
Foreign exchange gain (loss) |  | 17 | (15) | (15)
Gain on sale of assets and other |  | 9 | 46 | 1
Earnings (loss) before income taxes |  | (303) | 193 | (96)
Contribution from equity accounted investments |  |  |  |
Disclosure of operating segments [line items] |  |  |  |
Revenues |  | (3) |  |
Fuel, carbon compliance and purchased power |  | 0 |  |
Gross margin |  | (3) |  |
Operations, maintenance and administration |  | 0 |  |
Depreciation and amortization |  | 1 |  |
Asset impairment |  | 0 |  |
Taxes, other than income taxes |  | 0 |  |
Net other operating expense (income) |  | 0 |  |
Operating income |  | (2) |  |
Equity income from associate |  | 1 |  |
Finance lease income |  | 0 |  |
Net interest expense |  | 1 |  |
Foreign exchange gain (loss) |  | 0 |  |
Gain on sale of assets and other |  | 0 |  |
Earnings (loss) before income taxes |  | 0 |  |
Operating segments |  |  |  |
Disclosure of operating segments [line items] |  |  |  |
Revenues |  | 2,104 |  |
Fuel, carbon compliance and purchased power |  | 968 |  |
Gross margin |  | 1,136 |  |
Operations, maintenance and administration |  | 472 |  |
Depreciation and amortization |  | 655 |  |
Asset impairment |  | 84 |  |
Taxes, other than income taxes |  | 33 |  |
Net other operating expense (income) |  | (11) |  |
Operating income |  | (97) |  |
Equity income from associate |  | 0 |  |
Finance lease income |  | 7 |  |
Net interest expense |  | (239) |  |
Foreign exchange gain (loss) |  | 17 |  |
Gain on sale of assets and other |  | 9 |  |
Earnings (loss) before income taxes |  | (303) |  |
Hydro | Operating segments |  |  |  |
Disclosure of operating segments [line items] |  |  |  |
Revenues |  | 152 | 156 | 156
Fuel, carbon compliance and purchased power |  | 8 | 7 | 6
Gross margin |  | 144 | 149 | 150
Operations, maintenance and administration |  | 37 | 36 | 38
Depreciation and amortization |  | 28 | 32 | 30
Asset impairment |  | 2 | 2 | 0
Gain on termination of Keephills 3 coal rights contract |  |  | 0 |
Taxes, other than income taxes |  | 2 | 3 | 3
Termination of Sundance B and CPPAs |  |  | 0 | 0
Net other operating expense (income) |  | 0 | 0 | 0
Operating income |  | 75 | 76 | 79
Equity income from associate |  | 0 |  |
Finance lease income |  | 0 | 0 | 0
Wind and Solar | Operating segments |  |  |  |
Disclosure of operating segments [line items] |  |  |  |
Revenues |  | 332 | 312 | 282
Fuel, carbon compliance and purchased power |  | 25 | 16 | 17
Gross margin |  | 307 | 296 | 265
Operations, maintenance and administration |  | 53 | 50 | 50
Depreciation and amortization |  | 136 | 124 | 110
Asset impairment |  | 0 | 0 | 12
Gain on termination of Keephills 3 coal rights contract |  |  | 0 |
Taxes, other than income taxes |  | 8 | 8 | 8
Termination of Sundance B and CPPAs |  |  | 0 | 0
Net other operating expense (income) |  | 0 | (10) | (6)
Operating income |  | 110 | 124 | 91
Equity income from associate |  | 0 |  |
Finance lease income |  | 0 | 0 | 0
North American Gas | Operating segments |  |  |  |
Disclosure of operating segments [line items] |  |  |  |
Revenues |  | 217 | 209 | 232
Fuel, carbon compliance and purchased power |  | 66 | 74 | 96
Gross margin |  | 151 | 135 | 136
Operations, maintenance and administration |  | 49 | 44 | 48
Depreciation and amortization |  | 46 | 41 | 43
Asset impairment |  | 0 | 0 | 0
Gain on termination of Keephills 3 coal rights contract |  |  | 0 |
Taxes, other than income taxes |  | 2 | 1 | 1
Termination of Sundance B and CPPAs |  |  | 0 | 0
Net other operating expense (income) |  | 0 | (1) | 0
Operating income |  | 54 | 50 | 44
Equity income from associate |  | 0 |  |
Finance lease income |  | 5 | 6 | 8
Australian Gas | Operating segments |  |  |  |
Disclosure of operating segments [line items] |  |  |  |
Revenues |  | 158 | 160 | 165
Fuel, carbon compliance and purchased power |  | 10 | 9 | 8
Gross margin |  | 148 | 151 | 157
Operations, maintenance and administration |  | 32 | 37 | 37
Depreciation and amortization |  | 43 | 48 | 49
Asset impairment |  | 0 | 0 | 0
Gain on termination of Keephills 3 coal rights contract |  |  | 0 |
Taxes, other than income taxes |  | 0 | 0 | 0
Termination of Sundance B and CPPAs |  |  | 0 | 0
Net other operating expense (income) |  | 0 | 0 | 0
Operating income |  | 73 | 66 | 71
Equity income from associate |  | 0 |  |
Finance lease income |  | 2 | 0 | 0
Alberta Thermal | Operating segments |  |  |  |
Disclosure of operating segments [line items] |  |  |  |
Revenues |  | 619 | 816 | 912
Fuel, carbon compliance and purchased power |  | 573 | 570 | 666
Gross margin |  | 46 | 246 | 246
Operations, maintenance and administration |  | 131 | 138 | 171
Depreciation and amortization |  | 270 | 233 | 241
Asset impairment |  | 75 | 15 | 38
Gain on termination of Keephills 3 coal rights contract |  |  | 88 |
Taxes, other than income taxes |  | 15 | 13 | 13
Termination of Sundance B and CPPAs |  |  | 56 | 157
Net other operating expense (income) |  | (11) | (40) | (41)
Operating income |  | (434) | 31 | (19)
Equity income from associate |  | 0 |  |
Finance lease income |  | 0 | 0 | 0
Centralia | Operating segments |  |  |  |
Disclosure of operating segments [line items] |  |  |  |
Revenues |  | 497 | 571 | 442
Fuel, carbon compliance and purchased power |  | 279 | 416 | 314
Gross margin |  | 218 | 155 | 128
Operations, maintenance and administration |  | 60 | 67 | 61
Depreciation and amortization |  | 105 | 83 | 74
Asset impairment |  | 7 | (10) | 0
Gain on termination of Keephills 3 coal rights contract |  |  | 0 |
Taxes, other than income taxes |  | 5 | 3 | 5
Termination of Sundance B and CPPAs |  |  | 0 | 0
Net other operating expense (income) |  | 0 | 0 | 0
Operating income |  | 41 | 12 | (12)
Equity income from associate |  | 0 |  |
Finance lease income |  | 0 | 0 | 0
Energy Marketing | Operating segments |  |  |  |
Disclosure of operating segments [line items] |  |  |  |
Revenues |  | 122 | 129 | 67
Fuel, carbon compliance and purchased power |  | 0 | 0 | 0
Gross margin |  | 122 | 129 | 67
Operations, maintenance and administration |  | 30 | 30 | 24
Depreciation and amortization |  | 2 | 2 | 2
Asset impairment |  | 0 | 0 | 0
Gain on termination of Keephills 3 coal rights contract |  |  | 0 |
Taxes, other than income taxes |  | 0 | 0 | 0
Termination of Sundance B and CPPAs |  |  | 0 | 0
Net other operating expense (income) |  | 0 | 0 | 0
Operating income |  | 90 | 97 | 41
Equity income from associate |  | 0 |  |
Finance lease income |  | 0 | 0 | 0
Corporate | Operating segments |  |  |  |
Disclosure of operating segments [line items] |  |  |  |
Revenues |  | 7 | (6) | (7)
Fuel, carbon compliance and purchased power |  | 7 | (6) | (7)
Gross margin |  | 0 | 0 | 0
Operations, maintenance and administration |  | 80 | 73 | 86
Depreciation and amortization |  | 25 | 27 | 25
Asset impairment |  | 0 | 18 | 23
Gain on termination of Keephills 3 coal rights contract |  |  | 0 |
Taxes, other than income taxes |  | 1 | 1 | 1
Termination of Sundance B and CPPAs |  |  | 0 | 0
Net other operating expense (income) |  | 0 | 2 | 0
Operating income |  | (106) | (121) | (135)
Equity income from associate |  | 0 |  |
Finance lease income |  | $ 0 | $ 0 | $ 0

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